The present invention relates to an electromagnetic bearing for magnetically supporting a supported body in suspension, and more specifically to an electromagnetic bearing having a simple, compact structure which more efficiently uses its magnetic flux.
Electromagnetic bearings are conventionally known as bearings for supporting a supported body such as a rotating body, in which the supported body is levitated by magnetic force so that it is not in contact with any stationary part. Thus free from contact, these magnetic bearings provide various advantages, such as no mechanical friction, no mechanical noise, high-speed rotation, etc.
One such magnetic bearing is stated in, for example, Japanese Patent Disclosure No. 19844/83. In this magnetic bearing, as shown in FIG. 1, a cylindrical rotating body or supported body 12 is disposed in a case 10. A cylidrical member 14 of the supported body 12 is formed of a nonmagnetic material. A pair of ring-shaped members 16 is attached to the upper and lower portions of the inner peripheral surface of the cylndrical member 14. The ring-shaped members 16 are formed of a ferromagnetic material. Inside the supported body 12, a magnetic supporting means 20 is set on a base 22 at the bottom of the case 10. Two groups of magnetic poles 26 and 30 are formed on the upper and lower end portions, respectively, of a yoke 24 of the magnetic supporting means 20. Each group 26 or 30 includes four magnetic poles 26a, 26b, 26c and 26d (shown in FIG. 2) or 30a, 30b, 30c and 30d (only 30a and 30c are shown in FIG. 1) which protrude at a right angle to the longitudinal direction of the yoke 24 from the central portion thereof toward the ring-shaped members 16 and intersect it at right angles. The groups of magnetic poles 26 and 30 face the upper portion of the upper ring-shaped member 16 and the lower portion of the lower ring-shaped member 16, respectively. A ring-shaped magnetic pole 34 is formed on that portion of the yoke 24 which extends between the two groups of magnetic poles 26 and 30. The ring-shaped magnetic pole 34 faces that portion of the cylindrical member 14 which extends between the two ring-shaped members 16. Ring-shaped permanent magnets 39 and 40 are arranged on those portions of the yoke 24 between the one group of magnetic poles 26 and the magnetic pole 34 and between the other group of magnetic poles 30 and the magnetic pole 34. Coils 28a, 28b, 28c, 28d, 32a, 32b, 32c and 32d (32b and 32d are not shown in FIGS. 1 and 2) for adjusting the radial position of the supported body 12 are wound around the magnetic poles 26a, 26b, 26c, 26d, 30a, 30b, 30c and 30d, respectively. A pair of coils 36 and 38 wound around the yoke 24 is arranged individually on two flat surfaces of the ring-shaped magnetic pole 34. The coils 36 and 38 are used for adjusting the longitudinal position of the supported body 12. These coils are energized for controlling the following fluxes, and a magnetic flux delivered from the north pole of the permanent magnet 39 passes through the yoke 24 in loops indicated by the broken lines in FIG. 1, enters the ring-shaped members 16 via the groups of magnetic poles 26 and 34, and returns to the south pole of the permanent magnet 39. Likewise, magnetic flux delivered from the permanent magnet 40 which flux should be controlled by the energized coil returns thereto circulating in loops as indicated by the broken lines in FIG. 1. For example, magnetic flux from the magnetic pole 26a enters the upper ring-shaped member 16 substantially at right angles to the peripheral surface thereof. Therefore, if the energizing current of the coil 28a is increased to raise the magnetic flux density, the supported body 12 moves toward the magnetic pole 26a. Thus, the radial position of the supported body 12 can be adjusted by regulating the energizing currents of the groups of coils 28 and 32. Meanwhile, magnetic flux from the magnetic pole 34 enters the ring-shaped members 16 substantially at right angles to the lower and upper end faces of the upper and lower ring-shaped members 16. Accordingly, when the supported body 12 is located above a predetermined position, the energizing currents of the coils 36 and 38 are increased and decreased, respectively, so that the magnetic fluxes from the magnetic pole 34 which enter the upper and lower ring-shaped members 16 are intensified and weakened, respectively. Thus, the force of attraction between the upper ring-shaped member 16 and the magnetic pole 34 is increased to lower the supported body 12. The longitudinal and radial positions of the supported body 12 can individually be adjusted by regulating the coil energizing currents.
In the prior art electromagnetic bearing constructed in this manner, however, it is necessary that four magnetic poles be provided at each of the upper and lower portions of the yoke 24 to adjust and stabilize the supported body 12 in the radial direction, and that the magnetic pole 34 be provided for adjusting the longitudinal position of the supported body 12. Accordingly, this magnetic bearing requires a great number of magnetic poles and coils, and is complicated in construction and large-sized.
In another conventional electromagnetic bearing stated in U.S. Pat. No. 4,081,707, at least three electromagnets are arranged equidistantly on the circumference of a circle at each end of a rotating shaft. The electromagnets are positioned at an angle to the axis of the rotating shaft. In this case, a supported body is levitated by being subjected to a magnetic force in the axial and radial directions of the rotating shaft given by the slanted electromagnets. Thus, the required magnetic poles for the magnetic bearing may be reduced in number.
For fine positioning of the supported body in the axial or radial direction, however, it is necessary to adjust the coil energizing currents of a number of electromagnets. Accordingly, the supported body can be moved in one direction only after the energizing currents of a number of coils are calculated correlatively. This calculation is quite complicated; it is difficult to set the optimum current values and the control constant for position adjustment. In this magnetic bearing, the direction of the magnetic force acting on the supported body and the direction in which the supported body is to be moved are not coincident, intersecting each other at an angle of 90.degree. or less. Hereupon, the components of the magnetic force at right angles to the direction of the movement of the supported body match with and cancel each other. The presence of the magnetic force in the aforesaid perpendicular direction, however, leads to a waste of current for driving the electromagnets. In other words, even though the energizing current is increased to move the supported body, the magnetic force component applied in the direction of the movement of the supported body is smaller than the increment of the magnetic force caused by the increase of the current, so that the current efficiency is low. Since the supported body and the electromagnets are inclined, it is hard to increase structural accuracy, and the setting accuracy is low.